In many fluid systems, such as a hydraulic or air operated system, it is often necessary to provide a control valve for shutting off and/or exhausting flow downstream of the valve or for regulating the fluid pressure to a device downstream of the valve. A downstream device, such as a valve actuator, is thus controlled by the upstream valve.
In particular, it is often necessary in a fluid system to provide a valve controlling downstream flow which, in turn, is operated by a control device sensing a downstream condition, such as a pressure or other relevant condition. As an example, a condition monitoring device includes a block and bleed valve wherein flow through the block and bleed valve is resisted until a predetermined condition is encountered in the downstream flow. When this condition is present, the block and bleed valve opens to allow fluid flow therethrough. This flow is then used to actuate a control valve to shut off or regulate the downstream flow.
Typically, the block and bleed valve in a condition monitoring device is connected to the control valve to permit flow at either a preselected maximum or minimum fluid pressure in the downstream flow. In another type system, two block and bleed valves are used to control fluid flow outside a preselected range of fluid pressure. By way of example, block and bleed valves are controlled by a Bourdon tube monitoring a pressure or by a solenoid operated by a time controller. That is, a timer periodically operates the block and bleed valve to shut down or reset the control valve.